As creatine mono is a very safe supp and very cheap, Id assume the suggested 5g serving is effective, and I would suggest no less than 5g per day to ensure adaqueate phosphocreatine levels.
As for the high end of the spectrum... Creatine monohydrate is very safe, and the worst thing your going to do to yourself is two-fold:
1) waste money
2) your ass will explode all over the inside of your pants.
but your nowhere near that level yet.